1. Field of the Invention
The present invention relates to an arc tube for a discharge lamp unit, and more particularly to a mercury-free arc tube for a discharge lamp unit which does not contain mercury, but rather, contains main light emitting metal halide and starting rare gas enclosed in a closed glass bulb.
2. Description of the Related Art
FIG. 9 shows a discharge bulb which is a related-art discharge lamp unit used as a light source of an automotive lamp. The discharge bulb has a structure in which an arc tube 2 having a closed glass bulb 2a as a light emitting portion is integrated with an electrically insulating plug body 1 made of a synthetic resin. A rear end portion of the arc tube 2 is gripped by a metal support member 8 fixed to the electrically insulating plug body 1. A front end portion of the arc tube 2 is supported by a metal lead support 9 which serves also as a current conduction path extended out from the electrically insulating plug body 1.
The arc tube 2 has a structure in which main light emitting metal halide, buffer mercury and starting rare gas are enclosed in the closed glass bulb 2a which is held between pinch seal portions 2b and 2b located at opposite ends of the close glass bulb 2a, and which is provided with a pair of electrode rods 3 and 3 so as to be opposite to each other. Light is emitted on the basis of arc generated by electric discharge between the electrode rods 3 and 3. The discharge bulb is superior to that of an incandescent bulb because a large quantity of emitted light, a long lifetime, etc. can be achieved by the discharge bulb. For this reason, nowadays there is a tendency for this type of discharge bulb to be used as a light source for a head lamp or a fog lamp of an automobile.
The reference numeral 4 designates a lead wire led out from each pinch seal portion 2b. The reference numeral 5 designates a sheet of molybdenum foil for connecting the lead wire 4 to a corresponding tungsten electrode rod 3. Further, ultraviolet-shielding shroud glass 6 is integrally welded to the arc tube 2 to thereby form a structure in which the closed glass bulb 2a is surrounded by a closed space formed by the shroud glass 6. Hence, the inside of the closed glass bulb 2a is kept at a high temperature while ultraviolet rays in a wavelength range harmful to the human body are cut-off from light emitted from the arc tube 2.
Mercury enclosed in the related-art closed glass bulb 2a is a substance toxic to the environment. In response to the social needs of reducing the cause of global environmental pollution as much as possible, it is desirable that a mercury-free arc tube is developed.
The following findings have been obtained in the process of research and development on a mercury-free arc tube not containing mercury.
Mercury acts mainly as a buffer substance for keeping the tube voltage constant to reduce the amount of electrons colliding with electrodes to thereby buffer the damage of the electrodes. Therefore, when mercury is removed from the substances enclosed in the closed glass bulb, the tube voltage is reduced. That is, the tube power required for electric discharge cannot be obtained. It is therefore necessary to increase a tube current to increase the tube power. Thus, the load on each electrode increases (current density increases) correspondingly, so that the temperature of the electrode increases. With this temperature increase, the following problems occur.
First, the electrode is deformed by heat, or the glass bulb turns black near the root of the electrode by sputtering in the electrode surface, or the electrode is deformed because the chemical reaction between tungsten as an electrode constituent material and the enclosed substance (halide) proceeds in the electrode surface. Thus, the lumen maintenance factor (life performance) is lowered. In addition, the electrode is consumed so that the arc fades out. Further, the glass is cracked due to the difference in coefficient of thermal expansion between the electrode and the glass.